1. Field of the Invention
The present invention relates to a wire electrical discharge machine, and more specifically to a wire electrical discharge machine carrying out electrical discharge machining by inclining a wire electrode.
2. Description of the Related Art
In a case of carrying out machining with a wire electrical discharge machine, in a machining (step machining) in a step portion where a plate thickness of a workpiece suddenly changes, a streak of incision tends to generate in the step portion. This is considered to be generated by a rapid change of a fulcrum of the oscillation of the wire electrode in the step portion.
In a case where there is a step shape in the workpiece in wire electrical discharge machining, it is an important problem to suppress generation of a streak of incision in the step portion. Among the shapes of workpieces to be machined by wire electrical discharge machining, there are those having a step, and thus various control techniques have been proposed to suppress generation of such a streak of incision in the step portion. Specifically, such proposed technique includes prevention of a rapid change of a fulcrum of oscillation by inclining a wire electrode in a section sandwiching a step portion, control of energy applied from a power source for machining by detecting a step portion during the electrical discharge machining, and control of machining conditions in response to a change in the plate thickness.
A streak of incision generated in a step portion is described using FIG. 7.
In a case of machining a workpiece having a step portion where a plate thickness changes rapidly, a streak of incision is generated as shown by a reference numeral 51 in a plate thickness changing portion on a machining surface, while it is possible to suppress a streak of incision by inclining a wire electrode 4 when machining the step portion.
For example, in a first example disclosed in Japanese Patent Application Laid-Open No. 10-328938, whether or not a wire electrode is positioned in a section sandwiching a step portion is determined by setting the section sandwiching the step portion of a workpiece and an amount of inclination of the wire electrode. Then, in a case of determining that the wire electrode is positioned in the section sandwiching the step portion of the workpiece, the wire electrode is inclined at a predetermined angle to the workpiece based on the amount of inclination thus set to carry out electrical discharge machining. Then, the wire electrode is gradually returned from a position passing the step portion to the original state.
In the above technique, the section sandwiching the step portion is preset before machining, and in a case of determining that the wire electrode is positioned in the section sandwiching the step portion, the step portion is machined by inclining the wire electrode. Therefore, regardless of the number of machining, it is possible to machine by inclining the wire electrode to the step portion. However, since the section sandwiching the step portion has to be preset before machining, it turns out to take a larger number of steps for a setup.
In addition, in a second example disclosed in Japanese Patent Application Laid-Open No. 10-328938 above, by setting the amount of inclination of the wire electrode, the position of the step portion in the workpiece is detected from the state of machining the workpiece. Then, after detecting the position of the step portion, whether or not the wire electrode is positioned in the section sandwiching the step portion is determined, and in a case of determining that the wire electrode is positioned in the section sandwiching the step portion of the workpiece, the wire electrode is inclined at a predetermined angle based on the amount of inclination thus set to carry out electrical discharge machining, and the wire electrode is gradually returned from a position passing the step portion to the original state.
Since the above technique cannot incline the wire electrode in the step of detecting the step portion, it is assumed that the fulcrum of oscillation of the wire electrode rapidly changes when machining the step portion so that a streak of incision generates in the step portion.
Japanese Patent Application Laid-Open No. 2008-62303 discloses a control method to reduce the oscillation of the wire electrode for improvement of linearity of the machining surface of the workpiece and also to suppress the generation of a streak of incision in the step portion where the thickness of the workpiece rapidly changes. In this control method, when the state of electrical discharge in a machining gap is detected and electrical discharge does not occur in the machining gap even after a period of time defined in advance, it is determined that the machining gap is short circuited and thus the time to apply a pulse voltage for machining to the machining gap is shortened. This technique focuses on problems same as the present invention does, while it is different from the present invention which carries out control of machining the step portion by inclining the wire electrode, as described below, in that it controls the pulse voltage for machining to reduce the oscillation of the wire electrode.
Japanese Patent Application Laid-Open No. 2003-136339 discloses a technique that classifies modes of spraying a machining fluid from a machining fluid spray nozzle to a working gap into a plurality of types as differences in spraying conditions, and then, carries out machining by switching, for each spraying conditions thus classified, to a different plate thickness of an object to be machined and a set machining condition adapted to the machining of the plate thickness. However, this technique is different from the present invention in that a plate thickness of the object to be machined is determined from the mode of spraying of the machining fluid during electrical discharge machining to alter the machining conditions to those adapted to the plate thickness, which does not intend to carry out control for alteration of the machining conditions.
Japanese Patent Application Laid-Open No. 2011-16172 discloses a technique that detects a change of a plate thickness of a workpiece and controls energy applied from a power source for machining to decrease temporarily at a place where the plate thickness changes. It should be noted that, while this prior art technique focuses on the same problems as that of the present invention, the object of the prior art technique is to control the energy, not controlling an inclination angle of a wire electrode in a step portion of a workpiece, thus the present invention is different from the prior art technique in this respect.
Japanese Patent Application Laid-Open No. 2007-144567 discloses a technique that detects a change of a plate thickness of a workpiece in a rough machining step to store the plate thickness in association with a step position, and then, in a finish machining step after finishing the rough machining step, alters the machining conditions to machining conditions suitable for the plate thickness thus detected when a machining area reaches a region around the step position for machining with a constant electrical discharge gap. It should be noted that this technique stores a change in the plate thickness of the workpiece in rough machining step to alter the settings to machining conditions adapted to the change in the plate thickness in finish machining step, which is different from the present invention that carries out control of inclining the wire electrode in a step portion.
Japanese Patent Application Laid-Open No. 11-221718 discloses a technique that detects a change in the plate thickness of the workpiece and adjusts a length of a machining suspension time to suspend application of a voltage to a gap between the wire electrode and the workpiece using a plate thickness change rate, a pulse current peak value, a pulse width of the pulse current, and a set voltage and a feedback gain in servo feed control in which a feed back control is performed so that an average machining voltage in the gap between the wire electrode and the workpiece agrees with a set voltage, thereby regulating the energy applied from an electrical discharge current generator. However, this technique adjusts the energy applied from the electrical discharge current generator, such as electrical discharge suspension time and pulse current peak value, which is different from the present invention that controls the wire electrode so that it inclines a predetermined angle.
Japanese Patent Application Laid-Open No. 2007-290088 discloses a technique that calculates a plate thickness at a machining path in a workpiece based on numerical control data and three-dimensional data to define control details using the plate thickness. However, this technique calculates the plate thickness at the machining path in the workpiece from the three-dimensional data to feedback-control the energy of the voltage using control data corresponding to the plate thickness thus calculated, which does not incline the wire electrode at a predetermined angle, so that it is different from the present invention.